The present invention concerns a method for producing calcium carbonate from alkaline industrial waste or by-products, as well as for extracting calcium carbonate and vanadium from the mentioned waste or by-products.

Iron and steel manufacturing is one of the biggest industries in the world, producing annually more than one billion metric tons of steel. Large amounts of slag are produced as a by-product from iron- and steelmaking processes (annually 300-400 million metric tons worldwide). Current uses for steelmaking slag are cement aggregate, road construction, fertilizers, and liming material. The steel industry also accounts for approximately 6-7% of the total anthropogenic CO₂ emissions to the atmosphere. The slag does, however, contain many components, such as Ca, Si, Fe, Mg, Al, Mn, V, and Cr, which could be valuable when separated from the rest of the slag.

Synthetic calcium carbonate, or precipitated calcium carbonate (PCC), is today produced by three different processes: a lime-soda process, a calcium chloride process, and a calcination/carbonation process. In the lime-soda process, calcium hydroxide is reacted with sodium carbonate to produce a sodium hydroxide solution, from which the calcium carbonate is precipitated. In the calcium chloride process, calcium hydroxide is reacted with ammonium chloride, forming ammonia gas and a calcium chloride solution. After purification, this solution is reacted with sodium carbonate to form a calcium carbonate precipitate and a sodium chloride solution. In the third, and most commonly used, production process, calcium oxide is hydrated with water, producing a calcium hydroxide slurry. The slurry is reacted with a CO₂-rich flue gas, from which the calcium carbonate is precipitated.

The common PCC production processes require calcium oxide or -hydroxide as raw material, which is typically produced by calcining (i.e. burning) limestone, and causes significant CO₂ emissions. The virgin limestone used also needs to have low levels of impurities in order to affect the quality of the PCC.

The prior art contains some examples of processes for separating various components from alkaline industrial waste or by-products, such as iron- and steelmaking slag. An example is the separation of alkali metals from the slag by converting them into carbonates using carbon dioxide (CO₂) gas in water. This procedure is described, for example, in US 4225565, EP 0263559, US 5466275 and JP 57111215.

Further, JP 2005097072 provides a method for manufacturing an alkaline earth metal carbonate, wherein a gas containing carbon dioxide is contacted with an aqueous solution containing an alkaline earth metal and a salt of a weak base and a strong acid.

KR 20040026382 provides a method for manufacturing calcium carbonate using desulphurized slag and carbon dioxide or a CO₂-containing exhaust gas. In this method, the desulphurized slag is added to water, the pH is adjusted to ≥ 12 and the obtained calcium-eluted solution is reacted with CO₂ or a CO₂-containing exhaust gas under a pH of ≥ 7.

JP 2007022817 provides a method of treating a steelmaking slag, wherein CaO present in the steelmaking slag is carbonated using a CO₂-gas.

It is an aim of the present invention to provide means for efficient utilization of alkaline industrial waste or by-products and carbon dioxide (CO₂) -rich flue gases.

Particularly, it is an aim of the present invention to provide a method for extracting calcium and other metals from the mentioned waste or by-products using mild conditions, and a method for producing marketable calcium carbonate with lower carbon dioxide emissions than in many current calcium carbonate production methods.

These and other objects, together with the advantages thereof over known methods, are achieved by the present invention, as hereinafter described and claimed.

In the present invention, a new method for producing calcium carbonate is presented, which method eliminates the need for mining and burning limestone and makes use of alkaline industrial waste or by-products, such as iron- and steelmaking slags (including blast furnace slag, steel converter slag, desulphurization slag, and ladle slag). The method also has the potential to consume a substantial amount of the CO₂-containing flue gases from such industry. Further, it raises the concentration of valuable metals, such as vanadium, which can subsequently be extracted from the waste or by-product.

Thus, the present invention concerns a method for producing calcium carbonate, as well as for extracting calcium carbonate and vanadium from alkaline industrial waste or by-products.

More specifically, the method of the present invention is characterized by what is stated in Claim 1.

Considerable advantages are obtained by means of the invention. Thus, the present invention provides a new process, wherein low-cost industrial by-products can be used as raw materials instead of virgin limestone. Therefore, no mining or transportation of limestone is required. Further, the energy intensive limestone calcinations are omitted in this process, reducing CO₂ emissions.

The CO₂ concentration in the flue gases, for example, from an iron and steel plant can also be effectively reduced, reducing the local CO₂ emissions of the plant. For instance, by carbonating the iron- and steelmaking slags produced locally at steel mills, the carbon dioxide emissions of an individual steel mill could be reduced by 8-21 % (based on a study considering Finnish steel plants).

Also other valuable and/or toxic elements, such as vanadium, can be more easily extracted from the calcium-depleted residual product. This gives an additional benefit as valuable metals can be produced and toxic elements removed, which could make the residual product less harmful to the environment than fresh waste or by-product and better suitable for technical applications.

Next, the invention will be described more closely with reference to the attached drawings and a detailed description.

• Figure 1 is a process scheme presenting a preferred embodiment of the method of the present invention framed with a dashed, grey line.
• Figure 2 is a graphical presentation of the dissolution of a steel converter slag in ammonium acetate.
• Figure 3 is a graphical presentation of the dissolution of a steel converter slag in ammonium chloride.
• Figure 4 is a graphical presentation of the dissolution of a steel converter slag in ammonium nitrate.
• Figure 5 is a graphical presentation of the dissolution of a steel converter slag in ammonium dihydrogen phosphate.
• Figure 6 is a Scanning Electron Microscope (SEM) picture of a calcite produced at 30 °C using a solution prepared from CH₃COONH₄ and a steel converter slag.
• Figure 7 is a Scanning Electron Microscope (SEM) picture of a calcite produced at 30 °C using a solution prepared from NH₄NO₃ and a steel converter slag.
• Figure 8 is a Scanning Electron Microscope (SEM) picture of a calcite produced at 30 °C using a solution prepared from NH₄Cl and a steel converter slag.
• Figure 9 is a Scanning Electron Microscope (SEM) picture of a calcite produced at 70 °C using a solution prepared from NH₄NO₃ and a steel converter slag.

The present invention concerns a method for producing calcium carbonate from alkaline industrial waste or by-products, such as an iron- and steelmaking slag, while raising the concentration of valuable metals, such as vanadium, in the waste or by-product, which can subsequently be extracted therefrom.

The method of the present invention contains the steps of (Fig. 1):

1. a) Dissolution of alkaline industrial waste or by-products,
2. b) Filtration,
3. c) Carbonation of the residue, and
4. d) Filtration.

The dissolution step is carried out by extracting with an aqueous salt solution of a salt formed from a weak acid and a weak base. The weak base is preferably ammonia. Most preferably, the salt is ammonium acetate (CH₃COONH₄).

Using said steps of this method, calcium carbonate is obtained as a precipitate after the carbonation and the subsequent filtration.

Further steps are carried out according to the present invention, wherein the first residue obtained in the dissolution step a) is treated in order to extract vanadium. Thus, the method contains the steps of (Fig. 1):

1. a) Dissolution of alkaline industrial waste or by-products,
2. b) Filtration,
3. c) Carbonation of the first residue,
4. d) Filtration,
5. e) Dissolution of first residue, and
6. f) Filtration.

The dissolution step is carried out by extracting with an aqueous salt solution, which preferably is an aqueous solution of an ammonium salt, more particularly an aqueous solution of ammonium acetate (CH₃COONH₄), ammonium chloride (NH₄Cl) or ammonium nitrate (NH₄NO₃) or another ammonium salt, most preferably ammonium acetate (CH₃COONH₄).

The alkaline industrial waste or by-product is preferably iron- and steelmaking slag, more preferably blast furnace slag, steel converter slag, desulphurization slag or ladle slag of the iron- and steelmaking industry.

According to the process scheme of Figure 1, calcium is first selectively extracted from the alkaline industrial waste or by-products (step a), such as the iron- and steelmaking slag, using a first extraction solvent, at a temperature of 0-100 °C, preferably 10-70 °C, more preferably 10-30 °C. Most preferably, the extraction takes place at room temperature, specifically at 20-25 °C, using an aqueous salt solution. The salt concentration of the first extraction solvent is 0.2 to 5 M, preferably 0.5 to 2 M.

The extraction with ammonium acetate is presented in the following Equation 1:

        2CH₃COONH₄(aq) + CaO·SiO₂(s) + H_2O(1) → Ca²⁺ + 2CH₃COO^- + 2NH₄OH(aq) + SiO₂(s)     (Equation 1)

wherein the alkaline industrial waste or by-product has been simplified as CaO·SiO₂, although calcium (and many other compounds) may exist in several different phases in these products. Through the extraction of step a), a vanadium-enriched first residue, such as a residual slag, is formed, and allowed to settle from a formed calcium-rich first filtrate.

The first filtrate generally contains 60-90% of the calcium present in the waste or by-products, preferably 65-85%.

The solid first residue is allowed to settle and form a sediment, which is separated from the solution, i.e. the first filtrate, preferably by filtration (step b).

A carbonation gas is then led to a carbonating reactor, where the gas is preferably bubbled through the calcium-rich first filtrate, or the first filtrate is sprayed into a gas scrubber functioning as a carbonating reactor, whereby calcium carbonate is precipitated (step c). The carbonation takes place at a temperature of 0-100 °C, preferably 10-70 °C, more preferably 10-30 °C. Most preferably, the carbonation takes place at room temperature, specifically at 20-25 °C, using a gas, which preferably is CO₂ or a CO₂-containing gas, most preferably CO₂-containing flue gas of the steel industry, as presented in the following Equation 2:

        Ca²⁺ + 2CH₃COO^- + 2NH₄OH(aq) + CO₂(g) → CaCO₃(s) + 2CH₃COONH₄(aq)     (Equation 2)

The precipitated calcium carbonate is allowed to settle and form a sediment, which is filtered from the formed second filtrate (step d). The second filtrate that is separated from the formed calcium carbonate may be recycled to step a) to be used as the first extraction solvent.

A minor amount of the first extraction solvent, for example an ammonium acetate solution, the amount generally being 0.1-1 wt-% of the entire amount of first extraction solvent, evaporates with the CO₂-lean carbonation gas and is thus conducted away from the carbonation of step c) together with a stream of CO₂-lean gas. According to a preferred embodiment of the present invention, this solvent is regenerated, for example by condensing it using a condenser, and can thus be recycled to step a), either separately or together with the second filtrate, whereas the gas stream may be conducted back to the stack.

The yield of calcium precipitated as calcium carbonate after performing steps a) to d) is generally 20-35% of the calcium present in the waste or by-products, preferably 24-30%.

After the calcium extraction (steps a to d), the concentration of other elements in the first residue is higher. Using proper solvents, several other elements can be extracted as well. Since the first residue is enriched in, for instance vanadium, the extraction of vanadium is particularly advantageous. Vanadium can be extracted from the vanadium-enriched first residue by dissolving it into a second extraction solvent (step e), which preferably is ammonium dihydrogen phosphate, as shown in the following Equation 3:

        10NH₄H₂PO₄(aq) + V₂O₅(s) + 5H₂O(l) → 2V⁵⁺ + 10H₂PO₄^- + 10NH₄OH(aq)     (Equation 3)

wherein the vanadium in the alkaline industrial waste or by-product is simplified as V₂O₅, although it can be present in several different compounds. The salt concentration of the second extraction solvent is 0.2 to 5 M, preferably 0.5 to 2 M. The dissolution of the vanadium is carried out at a temperature of 0-100 °C, preferably 10-70 °C, more preferably 10-30 °C. Most preferably, the dissolution takes place at room temperature, specifically at 20-25 °C and results in the precipitation of a second residue, which is vanadium-lean, and in the formation of a third filtrate, which is enriched in vanadium. The precipitate and the filtrate may then be separated, for example by filtration.

The third filtrate contains 20-30% of the vanadium present in the industrial waste or by-product, preferably about 25%.

Metallic vanadium can be produced from this vanadium-enriched filtrate using, for example, electrolysis.

Although the preferable waste or by-product used in the present invention is iron- and steelmaking slag, other industrial residues or by-products containing one or more of calcium silicate, oxide and hydroxide are also potential raw materials for the process concept.

Experiments were performed to test the extraction of calcium from steel converter slag. Batches of 1g of the steel converter slag (74-125 µm) were dissolved in separate 50 ml aqueous solutions of respective ammonium salt solvent (acetate, nitrate and chloride) in concentrations of 0.5 M, 1 M and 2 M in sealed Erlenmeyer flasks. Each experiment was performed three times to increase the reliability and repeatability of the results. The solutions were stirred at 100 rpm at room temperature (20 °C), and 1 h after the addition of the slag filtered using 0.45 µm filters. The concentrations of Ca, Si, Fe, Mg, Al, Mn, V, and Cr (the main elements in steel converter slag) were measured in the filtered solutions with Inductively Coupled Plasma - Atomic Emission Spectrometry (ICP-AES). The steel converter slag fraction used was also analyzed using total digestion and ICP-AES, and a sample of the batch was sent for X-Ray Fluorescence (XRF) and X-Ray Diffraction (XRD) -analysis.

The results show that, while only 10% of the calcium in the steel converter slag was extracted using distilled water, a 2 M aqueous solutions of ammonium acetate managed to extract 69% of the calcium (Figure 2). Further, aqueous solutions of 2 M ammonium nitrate or -chloride were able to extract 82% and 73%, respectively, of the calcium in the steel converter slag (Figures 3 and 4). The obtained concentrations of other elements were low: only up to 6% of silicon was extracted and an amount below the XRF analysis detection limit of the other measured elements.

Using a 1 M aqueous solution of ammonium dihydrogen phosphate and maintaining the temperature at room temperature (20 °C), 25% of the vanadium in the steelmaking slag was extracted (Figure 5), leaving, however, most of the undissolved calcium. Further, 36% of the silicon dissolved into the solution, which means that the extraction of vanadium could be performed at elevated temperatures, such as temperatures of 60-80 °C, where dissolved silicon precipitates as silica gels, leaving a large concentration of vanadium in the solution. Therefore, using ammonium dihydrogen phosphate, selective extraction of vanadium is possible.

Carbonation of aqueous solutions containing ammonium salts and calcium dissolved from steel converter slag was tested at 30 °C and 70 °C. The experiments were performed in a glass reactor (1000 ml), which was heated by a temperature-controlled water bath. The reactor was connected to a tap-water condenser to prevent losses due to evaporation of solvent from the solution. The solution temperature and the pH were continuously monitored. The solution was stirred at 600-700 rpm using a magnetic stirrer. After heat-up to the desired temperature under nitrogen flow (bubbled through the solution at 1 l/min), the nitrogen flow was replaced with a carbon dioxide gas flow (1 l/min). After 70 minutes of continuous exposure to the carbon dioxide flow, the reactor was removed from the bath and the solution was filtered using 0.45 µm membranes. The precipitate was washed and dried at 115-120 °C overnight. The precipitates were analyzed using XRD, XRF, Total Carbon (TC) and Scanning Electron Microscope (SEM).

Soon after the nitrogen flow had been replaced with the carbon dioxide flow, the solution turned from clear to white. The XRD analysis of the washed and filtered precipitates showed that they consisted of calcium carbonate in the form of calcite. No other phase was visible in the XRD spectra. The XRF and TC analysis confirmed that the major elements of the precipitates were Ca and C. The sum of other components identified by XRF amounted to only 0.14-0.21 wt-% of the precipitates, indicating that the purity of the calcite produced was 99.8%. The SEM pictures of the precipitates (Figures 6, 7, 8 and 9) showed that the precipitates were in the form of rhombohedral calcite with a diameter of about 5-30 µm. Although the calcite favored for paper filler applications is typically smaller and has a scaleohedral (hexagonal) form, the form of the precipitate is typically adjusted by varying process parameters.

The total conversion of calcium in the slag to precipitated calcium was 28-29% using solutions prepared from NH₄Cl and NH₄Ac, while the conversion was 24% using a solution prepared from NH₄NO₃.